The core (kernel) of the two operating systems are different and an "upgrade" is not possible. You must do a clean install of Vista 64 to either an empty partition or a separate hard drive. Please understand that altering the stock configuration of your computer in this manner is a big change and gives your maker a legally viable reason to deny support. If you are technical enough to not need it, then fine. If not?? Well..
As far as licencing: Since your computer is OEM, your current licence key is not transferable. You will need to buy a separate copy of Vista 64 and use the key associated with that. Yes, buying another OEM copy would save you a few dollars.
As far as improvements: Compared to your current 32/Premium, 64 Ultimate will add backup/shadow copy, Fax capabilities, Remote Desktop, and a few other goodies. In my experience, drivers are *much* *much* less of an issue than they were a year ago - The caveat being if you have old accessories around. Make sure those have drivers, or be prepared to replace them. Also, Shareware/freeware which is not digitally signed will NOT get past Vista 64's security. So if you habitually use such programs, be advised you will no longer be able to do so in many cases. There are some workarounds, but no permanent alternative short of editing the registry.
The other minor issue is there is no 64 bit version of Flash. So if you use a 64 bit browser, Flash based content will not work at all. Having said that - You can run a 32 bit Browser (using Firefox ATM), with Flash installed on it, and that works perfectly.
I went to 64 in stages - Dual booting with 32 bit Vista for a short period of time. In and of itself, Vista 64 is a bit quicker and perceptibly snappier than 32. Not an "Oh My Gawdd" difference. But noticable just the same. I update/upgrade my software as a matter of long habit, and have had no issues with any purchasable programs. Older items can be run in various selectable compatibility modes, but by and large everything just runs.
From a reliability perspective, it is my experience that Vista 64 is more solid and stable than 32. NO program crash has ever taken down V64 since I've been running it. I cannot say the same for V32 or XP32. There is a bit of a down side tho - In the case of a bad program or driver, the lack of Blue Screen codes can make troubleshooting more difficult.
From a user perspective, Vista 32 and Vista 64 are almost exactly the same. Most people would only notice the fact you have two versions of Internet Exploder instead of one, and if they looked they'd see the version installed is 64 bit.
